<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script src="/js/vue.js"></script>
    <div id ="app">
       <p>{{msg}}</p>
    </div>
    
<script>
    let app =new Vue({
        el:"#app",
        data: {
            km: 0,
            m: 0
        },
        methods: {
            kmtom: function (val) {
                this.m = this.km * 1000;
            },
            mtokm: function (val) {
                this.km = this.m / 1000;
            }
        },
        watch: {
            km: 'kmtom',
            m: 'mtokm'
        }
    })
</script>  
</body>
</html>